Department of Primatology

Gorillas (conducted by Martha Robbins)

Gorillas are fascinating animals to study because they live in a wide variety of environments, they have a flexible social system, and they are one of our closest living relatives. Gorillas live in a broad diversity of habitats in ten Central African countries, ranging from coastal lowland forests to the high altitude, afromontane rainforests. They live in cohesive social groups that typically consist of one or more silverback (adult male), several adult females, and immature offspring. There are two species of gorillas, western gorillas (Gorilla gorilla) and eastern gorillas (Gorilla beringei). Each species has two subspecies. Despite their wide distribution, all four subspecies of gorillas are critically endangered, meaning they are at risk of going extinct. A major goal of the gorilla group at the MPI-EVAN is to understand the causes and consequences of variability in ecology, behavior, life histories, and population dynamics of gorillas.

Our main project is the Bwindi Gorilla Project, located in Bwindi Impenetrable National Park, Uganda, where we have been researching mountain gorillas since 1998 [see more about the Bwindi Gorilla Project here]. This is the second longest running research project on habituated gorillas. Our other main project is the Loango Gorilla Project, studying western gorillas in Loango National Park, Gabon [see more about the Loango Gorilla Project here]. This project has been running since 2005 and we have been making systematic behavioral observations one group of habituated gorillas since 2014 [see more about Loango here]. We have collaborative projects with several other field sites including the Karisoke Research Center run by the Dian Fossey Gorilla Fund International and the Mbeli Bai Project in the Republic of Congo run by the Wildlife Conservation Society. Lastly, we run a conservation education program (BACEP: Bwindi Ape Conservation Education Partnership in collaboration with the North Carolina Zoo and Cleveland Metroparks Zoo), working with four primary schools located on the edge of Bwindi [see more about BACEP here].

Specific Research Topics Include:

Social Behavior

Living in social groups involves both costs and benefits revolving around feeding competition, cooperation, affiliation, and reproductive strategies. Gorillas are particularly interesting species to examine the causes and consequences of social behavior because of the variability environments they inhabit, the high level of reproductive competition among males, and their variable grouping patterns. The main research questions we address are: How do gorillas cope with the various costs and benefits of living in cohesive social groups? How do differences in diet influence the social relationships and nutritional intake of group members? What reproductive strategies are used by males and females?

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Our group has tested basic principles of socioecological models, notably how the distribution and abundance of food resources influences female social relationships (Grueter et al., 2016; Robbins, 2008; Wright & Robbins, 2014; Wright et al., 2014). Our studies on feeding competition, dominance rank, and food intake of adult female gorillas using similar methodologies in the Virungas and Bwindi suggest that even in primates feeding primarily on abundant herbaceous vegetation, high rank confers priority of access to relatively valuable food patches (Grueter et al., 2016; Wright & Robbins, 2014).We study the reproductive strategies of gorillas, particularly focusing on factors that may influence why gorillas live in both one-male and multimale groups. We have examined male-female social relationships to elucidate if male agonistic behavior is coercive or simply courtship (Robbins, 2009; Breuer et al., 2016; Habumuremyi et al., in review). We have combined behavioral observations with endocrine measures to study female reproductive behavior (Habumuremyi et al., 2016). We have used genetics to determine paternity of offspring and estimate reproductive skew in males (Bradley et al., 2005; Vigilant et al., 2015) as well as agent-based modelling to estimate the fitness consequences of dispersal versus philopatry (Robbins & Robbins, 2005). We currently are studying how male-male aggressive interactions are influenced by body size (measured with photogrammetry), which will help us understand male reproductive success and the variable social system of gorillas.We plan to maximize the use of the nearly 20 years of long-term data from Bwindi to study topics including affiliation and social bonds. To understand how development influences sociality, we are examining the ontogeny of feeding behavior and social relationships in immature gorillas. We also conducted a comparative study of gorilla behavior at five field sites and concluded that some behavioral variability in both species of gorillas may be due to potential cultural traits (Robbins et al., 2016).

Life History Strategies

The gorilla group focuses on the factors that shape the variability in life history events, grouping patterns, and reproductive success in gorillas. Gorillas are interesting species to examine life history strategies because they have the fastest reproductive rates of any extant ape and both males and females have flexible dispersal patterns. Through various collaborative projects, we have worked with demographic datasets for three populations of gorillas that each span more than 20 years with each containing more than 300 individuals: the Virunga Volcanoes, Bwindi, and Mbeli Bai.The overarching questions we address are: How do individual behavioral and demographic factors (eg. dispersal patterns, group size, birth and mortality rates) influence individual reproductive success?How do individual life history variables and environmental factors translate into population level phenomena (e.g. population structure and growth rates)?

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We have conducted several studies on age, dominance rank, and longevity that lead to variation in reproductive success of both males and females in both species of gorillas (Breuer et al., 2010, 2012; Nsubuga et al., 2008; Robbins et al., 2006, 2007, 2011; Vigilant et al., 2015). The interval between surviving births of Bwindi mountain gorillas is five years compared to four years in the Virunga mountain gorillas (Robbins et al., 2009). Despite this difference, the age of first reproductive for females is similar, but males appear to reach maturity at a later age in Bwindi, suggesting that there may be differences for males and females in energetics and/or life history trade-offs between growth and reproduction. Analysis of these parameters for western gorillas is underway to enable a broader comparative perspective. We also are conducting a study of body size and growth trajectories for a comparison of Bwindi and Virunga mountain gorillas in collaboration with Drs. Shannon McFarlin and Jordi Galbany of George Washington University and DFGFI.Male dispersal is universal in western gorillas, but only about 50% of male mountain gorillas disperse. As a result, multimale groups are common in mountain gorillas but almost never occur in western gorillas. Several factors may be responsible for this difference including variation in group size, risk of infanticide, and the presence of mothers (Robbins et al., 2013, 2016). We also have ongoing studies comparing population dynamics on a larger spatial scale via long-term genetic sampling to estimate changes in population size, structure, and individual group movements over a 10 year period in both Loango and Bwindi [building on Arandjelovic et al.,2010; Roy et al., 2014).

Feeding Ecology

Food resources vary in time and space, resulting in animals needing to adopt strategies to maximize their foraging efficiency within changing environments. How animals respond to the abundance and distribution of food resources in their environment has implications for many key elements related to survival and reproduction including patterns of habitat utilization, group compositions, and social interactions, and population dynamics.How do differences in food availability influence variability in the diet and nutritional ecology of gorillas living in different habitats?How are patterns of habitat utilization influenced by food availability and dietary patterns?

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Research done by the gorilla group has shown that there is large variability in the food availability and diet of gorillas, and most notably, that gorillas incorporate a significant amount of fruit in their diet in all locations except the Virunga Volcanoes (e.g. Ganas et al., 2004; Head et al., 2011; Masi et al., 2015). Comparative studies on food availability, dietary intake and energy balance at three locations have advanced our understanding of the relationship among these factors in differing environments (Wright et al., 2014; Greuter et al., 2016; Head et al., 2011). We have analysed macronutrients to estimate energy content of food, and using C-peptides have shown that Karisoke gorillas, living in a seemingly aseasonal environment, exhibit seasonal variation in energy balance that is related to bamboo consumption (Grueter et al., 2014).Variability in food availability determines patterns of habitat utilization, which is linked to between-group competition and population dynamics. A study on ecological and social influences on space use in 10 groups of Bwindi gorillas has shown that the gorillas alter their movement patterns on daily, monthly and annual scales according to food availability, even in environments with highly abundant food resources (Seiler et al., in review). Furthermore, social factors (group size, density of groups, and intergroup encounters) influence movement patterns. Interestingly, we showed that non-territorial gorillas have overlapping home ranges but maintain largely exclusive core areas, which may reduce intraspecific competition for both food and mates. These findings emphasize that within and between-group interactions are driven by both social and ecological causes.Collaborative projects have also included examining niche partitioning among sympatric chimpanzees and gorillas using indirect measures of diet (Ganas et al., 2004), remotely triggered camera traps (Head et al., 2012), and stable isotopes (Oelze et al., 2014, 2016). An expansion of our previous work on feeding ecology will include examining processes of spatial cognition as well as making direct comparisons of patterns of dietary intake and habitat utilization between mountain and western gorillas.